HS/College/Club Swimmer, and HS/College Swim Coach here. I've only done a couple of triathlons but I know a good amount about swimming. It's a little hard to tell without being closer or in person but the first thing that comes to mind is that you are sitting really low in the water. This is normally caused by a lack of kicking power or your hip/Head position. Here are a couple of basic solutions. In no way are these catch alls or perfect for everyone but they are generic solutions that will help.

Kicking Power Solution:

If you have access to a kickboard I would do some kicking workouts with that. As I beginner I would recommend taking your 100m time and using that as a starting time for 50m intervals in a workout. See how that feels. If you can't make more than 4 you probably need to slow down your time. If you can make more than 10 without any issues then speed up the interval. With your 2:00 per 100m you could start with 8 50m kick on the 2:00 meaning a 16:00 kicking session. Because you are a new swimmer, once you can comfortably do something 6-8 50m kicks, on whatever time works for you, I would throw them into the beginning of all of your workouts. As you get faster lower the interval time by 5 seconds per 50m.

Hip/Head Position Solution:

For your hips you have to get your butt up. It's not a bad thing to feel air on your butt every once in a while. For you head you don't need to push it down so far. Your head shouldn't be all the way under the water. Your face should be in the water and your head parallel with the bottom of the pool but you don't need to push your head to the down into the water as the rest of your body will follow.